The invention relates to liquid crystal displays.
Liquid crystal displays (LCDs) are widely used in information displays. Due to the intrinsic optical anisotropy of liquid crystal display materials, the incident light that transmit from different directions can produce different effective birefringence. Therefore, the viewing angle of conventional LCDs is not as wide as in self-luminescent displays, such as cathode-ray tubes (CRTs), organic light-emitting diodes (OLEDs) and plasma display panels (PDPs).
In order to widen the viewing angle, several display modes using lateral electric field to activate the LC molecules, such as in-plane switching (IPS) mode and fringe field switching (FFS) mode have been proposed. In both IPS and FFS modes, the LC molecules at voltage-off state are basically homogeneously aligned on the glass or plastic substrates that are coated with a thin indium-tin-oxide (ITO) layer and then overcoated with a polyimide alignment layer. The surfaces of the polyimide layers are rubbed in an anti-parallel direction to create homogeneous alignment. The display panel is sandwiched between two crossed polarizers, and the long axis of LC molecules is either parallel or perpendicular to the transmission direction of their adjacent polarizers. At on-state, these LC molecules can twist in the plane parallel to the supporting substrates by the lateral electric field generated from the comb-shaped electrodes.
Light efficiency is proportional to the total retardation change experienced by the incident light traveling in the liquid crystal layer of the device. The total retardation change is a product of 1) birefringence Δn, of the liquid crystal molecules and 2) total path length traveled by the incident light in the liquid crystal layer. FIG. 1 is a schematic view of a conventional multi-film compensated liquid crystal display. In FIG. 1, a liquid crystal display 1 comprises a first substrate 10, a second substrate 20, and a liquid crystal layer 30 sandwiched between the first substrate 10 and the second substrate 20. A first polarizer 40 is laminated outside the first substrate 10 opposing the liquid crystal layer 30. A second polarizer 50 is laminated outside the second substrate 20 opposing the liquid crystal layer 30. A first birefringence compensation film 15 is interposed between the first polarizer 40 and the first substrate 10. A second birefringence compensation film 25 is interposed between the second polarizer 50 and the second substrate 20.
Conventional transflective liquid crystal display uses a single compensation film to compensate the birefringence of the liquid crystal layer. Alternatively, some liquid crystal displays use two compensation films with the same birefringence polarity to compensate the birefringence of the liquid crystal layer.
FIG. 2A is a schematic view of a conventional compensation film with negative birefringence polarity (Δn<0). The compensation film with negative birefringence polarity (Δn<0) comprises disk-like liquid crystal molecules 22. FIG. 2B is a schematic view of a conventional compensation film with positive birefringence polarity (Δn>0). The compensation film with positive birefringence polarity (Δn>0) comprises rod-like liquid crystal molecules 24. Conventional compensated liquid crystal uses the same birefringence polarity films to compensate the birefringence of the liquid crystal layer. Since the first compensated film and the second compensated film have same birefringence polarities, the birefringence therebetween cannot be compensated, causing a narrow viewing angle in normally white displays.